runtime: fix signal handling on Plan 9

Once added to the signal queue, the pointer passed to the
signal handler could no longer be valid. Instead of passing
the pointer to the note string, we recopy the value of the
note string to a static array in the signal queue.

Fixes #10784.

Change-Id: Iddd6837b58a14dfaa16b069308ae28a7b8e0965b
Reviewed-on: https://go-review.googlesource.com/9950Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/sigqueue_plan9.go

@@ -17,21 +17,29 @@ var sig struct {
 	sleeping bool
 }
 
+type noteData struct {
+	s [_ERRMAX]byte
+	n int // n bytes of s are valid
+}
+
 type noteQueue struct {
 	lock mutex
-	data [qsize]*byte
+	data [qsize]noteData
 	ri   int
 	wi   int
 	full bool
 }
 
+// It is not allowed to allocate memory in the signal handler.
 func (q *noteQueue) push(item *byte) bool {
 	lock(&q.lock)
 	if q.full {
 		unlock(&q.lock)
 		return false
 	}
-	q.data[q.wi] = item
+	s := gostringnocopy(item)
+	copy(q.data[q.wi].s[:], s)
+	q.data[q.wi].n = len(s)
 	q.wi++
 	if q.wi == qsize {
 		q.wi = 0
@@ -43,14 +51,15 @@ func (q *noteQueue) push(item *byte) bool {
 	return true
 }
 
-func (q *noteQueue) pop() *byte {
+func (q *noteQueue) pop() string {
 	lock(&q.lock)
 	q.full = false
 	if q.ri == q.wi {
 		unlock(&q.lock)
-		return nil
+		return ""
 	}
-	item := q.data[q.ri]
+	note := &q.data[q.ri]
+	item := string(note.s[:note.n])
 	q.ri++
 	if q.ri == qsize {
 		q.ri = 0
@@ -86,8 +95,8 @@ func sendNote(s *byte) bool {
 func signal_recv() string {
 	for {
 		note := sig.q.pop()
-		if note != nil {
-			return gostring(note)
+		if note != "" {
+			return note
 		}
 
 		lock(&sig.lock)